By adopting the minimum value for Vesta’s
global average [H] and ignoring layering, [H]
can be mapped as a lower bound ( 14). The spatial variation of H on Ceres can be estimated
with a total uncertainty of less than 1 wt WEH
from thermal plus epithermal neutron rates ( 10).
The resulting map of [H] varies strongly with
latitude and is asymmetric, with higher [H] in
the north than in the south (Fig. 3A). The difference between the poles (2.5 ± 1.1 wt WEH)
is larger than predicted by ice-stability models,
given Ceres’ precessing orbital elements. Epithermal and fast neutron maps are also asymmetrical ( 10), which implies subtle differences in
composition and/or layering between the two
hemispheres.

Based on forward modeling of zonally averaged thermal plus epithermal counting rates ( 10),
water ice approaches the surface above 40° latitude in both hemispheres (Fig. 3). The spatial
distribution of OH and ammoniated phyllosilicates measured by VIR at up to 60° latitude
is relatively uniform ( 12). Thus, the latitude variation of GRaND measurements is probably not
attributable to changes in the abundance or
hydration state of minerals within the uppermost surface layer. An estimate of [H] in the ice
table can be obtained by subtracting the equatorial [H] from the values measured at the poles.
Using this approach, we find that Ceres’ ice table
contains about 10 wt water ice, given that the
ice table is within a centimeter of the surface at
the poles (Fig. 3, B and C). The calculated porosity of the regolith is 0.2, assuming the ice fills the
pores and the grain density is 2.5 g/cm3, representative of CI and CM chondrites ( 15).

4. 5 billion years after formation, calculated for
two cases considered in ( 6): a grain size of 1 mm
and porosity of 0.1 (low diffusivity, case a) and
a grain size of 10 mm and porosity of 0.5 (high
diffusivity, case b). A third case representing our
lower-bound porosity of 0.2, with a 1-mm grain
size, is also shown (low diffusivity, case c). The
latitude depth profile of case c is similar to that
of case a. Forward models of thermal plus epithermal counts are compared with the data in
Fig. 3C. The low-diffusivity case c most closely
matches the data.

The steady-state water vapor emission rate implied by the models (0.003 kg/s for case c) is much
lower than the 6 kg/s inferred from Herschel
Space Observatory data ( 16). Neither the episodic
emissions observed by Herschel nor a possible
transient atmosphere observed by GRaND during the Survey orbit mission phase ( 17) can be
explained by sublimation of subsurface ice. Hence,
the Herschel observations may relate to the temporary exposure of ice on the surface.

Measurements of H and Fe provide constraints
on the abundance of aqueous alteration products,
assuming that the mineralogy is similar to carbonaceous chondrites ( 10). The average equatorial
[H] of 17 ± 2 wt WEH, which is representative
of Ceres’ non-icy regolith, is similar to measured
upper limits for CM and CI chondrites of about
14% WEH ( 18). These meteorites contain up to
a few weight percent organic material, with a
H/C ratio of close to 0.7 ( 19). An organics feature was not identified in the VIR global spectrum of Ceres (1). Although the presence of
organics cannot be excluded ( 20), we expect
that hydrogen is partitioned primarily between
phyllosilicates and water ice on Ceres. The
equatorial average [Fe] was determined to be
16 ± 1 wt from the Fe 7.6-MeV interaction
rate and thermal plus epithermal neutron measurements ( 10).

Aqueous alteration results in the separation of
feed materials into a briny liquid and less-mobile
solid residue. Elements such as Fe would be
found primarily in the solid phase, whereas K
and C are partially soluble and could be transported in the brine. The CI and CM chondrites
underwent low-temperature aqueous processing,
probably in a low-permeability environment on a
small parent body, where fluid flow was limited
( 21, 22). This led to isochemical alteration ( 8). On
larger bodies like Ceres, multikilometer transport is possible, potentially resulting in ice-rock
fractionation (2– 4, 8, 21).

Analysis of the 1.461-MeV gamma ray produced
by the decay of 40K ( 10) yields an equatorial
concentration for K of 410 ± 40 mg/g, intermediate between the CI and CM average values
[550 and 370 mg/g, respectively ( 23)]. However,
[K] depends on both nebular and parent-body
processes ( 24). Without knowledge of Ceres’
bulk [K], the regolith measurement is not diagnostic of chemical fractionation within Ceres.
Carbon was detected by GRaND with a lower
limit similar to that of carbonaceous chondrites ( 10).

If carbonaceous chondrite alteration was isochemical ( 8), the CI and CM chondrites would
follow a water-dilution trend when [Fe] is plotted against WEH (Fig. 4). The [Fe] measured by
GRaND is lower than the level of CI chondrites.
This may result from differences between the bulk
composition of the meteorite parent bodies and
Ceres. Low regolith vapor diffusivity inferred by
GRaND is consistent with small grains, which
may be representative of primordial materials ( 8).
Alternatively, internal processes, such as density-driven separation of Fe-rich phases from a primitive starting composition could explain the
difference. Convective upwelling, possibly involving brines, could have led to the exposure
of ice and fine grains on the surface ( 4). Coarse
magnetite grains are found in CI and CM chondrites ( 25). Consequently, sedimentation could result in a surface depleted in Fe. Otherwise, Fe may
be diluted by organic materials, carbonates, or
salts present in Ceres’ crust ( 7, 10, 26). Systematic
uncertainties may also contribute to differences
between Ceres and the meteorites (Fig. 4).

Together, measurements of [Fe], [H], and [K]
show that materials exposed on Ceres’ surface
have undergone aqueous processing. Evidence
from GRaND for a fine-grained, C-bearing regolith, combined with differences between regolith [Fe] and [H] relative to CI chondrites, implies
modest ice-rock fractionation. The high spatial
uniformity of GRaND measurements of the ice-free, equatorial regolith indicates that ice-rock
fractionation occurred on a global scale. The [H]
within Ceres’ icy regolith, around 27 wt WEH,
is consistent with estimates of bulk H from Ceres’
density (1, 2). The ice table contains about 10 wt %
water ice, most likely sourced from endogenic
liquid not consumed by alteration processes. The
detection of widespread water ice at mid-to-high
latitudes confirms predictions that ice can survive for billions of years within a meter of the
surface.

Fig. 4. Fe versus water-equivalent H for the
CI and CM chondrites
and Ceres. Recent measurements of H via mass
spectrometry and thermo-gravimetric analysis (TGA)
are shown. See ( 10) for data
sources. Water dilution
trends, in which [Fe] is
adjusted by adding or
removing water, are displayed for the CI chondrites.

Ceres’ equatorial average
is shown as a point with
error bars indicating total
uncertainty ( 10). The
decrease in Fe on Ceres
relative to the CI chondrite
trend is consistent with the addition of 13 wt of a neutral component. The analysis assumes the
minimum for the average [H] on Vesta of 250 mg/g. Increasing the hydrogen content of Vesta’s regolith
causes Ceres’ regolith composition to shift toward the dilution trend for CI chondrites (light blue line).